A point on the screen you're
looking at (like this red one: .) has a "location" which
is measured by how many pixels across and down it is from the
upper left corner. These are its "screen coordinates."

In math, the coordinates of
a point in the plane are measured in relation to a "central"
point, the origin, first to the right, then up.

The coords are listed as (x,
y) for (over, up). In the picture, the origin is at the + and the red dot has coords (x, y) = (5, 2).